Electrochemical cells and systems, for instance rechargeable batteries and fuel cells, play a crucial role in a variety of applications such as energy storage, conversion and management; in particular they can be employed to address grid stability issues in conjunction with both renewable and nonrenewable energy sources.
Among rechargeable batteries, flow batteries represent a particularly advantageous solution for the applications above because of their limited costs, high efficiency, modularity, and easy transportability.
Flow batteries usually comprise a plurality of electrochemical cells and provide energy storage via one or more electroactive compounds dissolved into liquid electrolytes. In zinc bromine flow batteries, for example, an aqueous solution of zinc bromide is stored into two tanks and can be circulated through the system. During the charge cycle of the battery, metallic zinc is electroplated from the electrolyte solution at the surface of the negative electrode, while bromine is formed at the positive electrode. On discharge the reverse process occurs: bromine is reduced to bromide while the metallic zinc dissolves back into the solution, where it remains available for the next charge cycle of the battery.
Advantageously, zinc bromine flow batteries can be left completely and indefinitely discharged without damage; they have no practical shelf life limitations, and provide high cell voltage and energy density compared to other types of flow batteries.
The growing interest in the electrochemical cells and systems described above translates into ongoing efforts aimed at optimizing these devices in terms of cost, efficiency and life-time, and also with respect to their potential impact in connection with environmental and health and safety issues.
In this respect, the optimization of the electrodes where the electrochemical reaction occurs is key in improving the overall performance of the systems that implement them. A parameter that can be usefully employed to assess the efficiency of rechargeable electrochemical cells is the voltage efficiency, defined as the ratio between the average discharge voltage and the average charge voltage of the cell, expressed in percentage. The voltage efficiency is therefore a function of the energy required by the system for charging during storage operations, on one side, and of the energy released by the system during discharge, on the other. The higher the voltage efficiency of the cell, the more convenient it is in terms of operating costs and energetic performance.
In a zinc bromine flow battery, it is possible to achieve and maintain voltage efficiency above 66% using a metal electrode coated with a catalytic composition comprising high molar percentages of one or both iridium and platinum, for example 70% and 23% respectively along with other catalytic metals. Since platinum is believed to foster the bromine reduction reaction, it is not surprising that a coating with relatively high platinum content should exhibit satisfactory performances in terms of voltage efficiency, as it favorably impacts on the energy of the discharge process of the cell.
However, both iridium and platinum are particularly expensive materials and their price greatly impacts on the production costs of the electrodes.
Furthermore, hazards of occupational exposure to select platinum containing compounds include respiratory and skin ailments that require stringent exposure limits and/or limit the daily manufacturing capacity of a production facility. In the field of catalytic converters for use in conversion of automotive pollutants, platinum is typically the abundant noble metal in the catalyst formulation. Therefore a low yet effective platinum concentrated coating, which boosts performance for the bromine redox reaction in zinc bromine flow batteries, for example, and limits platinum exposure and associated ailments during handling, is an added benefit.
Additionally, any battery or electrolysis process that employs metal plating/de-plating on an electrode surface (such as a Zn-halogen battery) benefits from low levels of metallic impurities. Such impurities incorporated into the electrochemical process can lead to non-uniform metal plating, growth of metal dendrites and shortened cell life. Metal contaminated electrolyte can stem from dissolution of mixed metal oxide-coated substrate, which has shown to be particularly prevalent in the case of certain platinum containing coatings. For example, early investigations into Pt—Ir (70:30 wt % ratio) mixed metal oxide coatings for electrowinning revealed preferential platinum dissolution over iridium (D. Wensley and H. Warren, “Progressive Degradation of Noble Metal Coated Titanium Anodes in Sulfuric Acid and Acidic Copper Sulfate Electrolytes”, Hydrometallurgy, 1 (1976), pp. 259-276.; D. Wensley and I. H. Warren, “Corrosion and Passivation Behavior of Noble Metal Coated Anodes in Copper Electrowinning Applications,” Metall. Trans. 6. 1OB (1979), pp. 50S511). Mixed metal oxide corrosion can also be accelerated by organic additives. Organic complexing agents are often introduced in electrolytes containing bromine in order to control the solubility of this volatile component. Maintaining low yet effective weight ratios of platinum in the mixed metal oxide matrix minimizes potential risks related to the release of platinum impurities in the electrolyte. This is beneficial to a battery system that is expected to require little to no maintenance over its lifetime and provide consistent performance for more than 10 years.
Therefore, the parameters that should be taken into account in the design of electrodes for electrochemical cells, for instance in energy storage applications, are both those defining the performance of the electrode (such as the voltage efficiency, current density, stability and lifetime) and those impacting on costs and safety issues (such as the costs of the raw materials constituting the electrode, and the costs related to the management and disposal of possible hazardous materials employed in the fabrication process). All these parameters affect the overall economics of the system and should be globally optimized.
It is therefore desirable to produce an electrode for electrochemical cells provided with a suitable catalytic coating composition that allows achieving high voltage efficiencies, possibly above 70% and preferably above 73%, and good stability for operating lifetimes above 10 years. Such catalytic coating composition should also minimize the costs of the raw materials and the amount of hazardous substances employed for its preparation, without compromising, and possibly enhancing, the efficiency and duration of the electrode.